Cobalt-based alloys with γ/γ′ microstructures have the potential to become the next generation of superalloys, but alloy compositions and processing steps must be optimized to improve coarsening, creep, and rafting behavior. While these behaviors are different than in nickel-based superalloys, alloy development can be accelerated by understanding the thermodynamic factors influencing microstructure evolution. In this work, we develop a phase field model informed by first-principles density functional theory and experimental data to predict the equilibrium shapes of Co-Al-W γ′ precipitates. Three-dimensional simulations of single and multiple precipitates are performed to understand the effect of elastic and interfacial energy on coarsened and rafted microstructures; the elastic energy is dependent on the elastic stiffnesses, misfit strain, precipitate size, applied stress, and precipitate spatial distribution. We observe characteristic microstructures dependent on the type of applied stress that have the same γ′ morphology and orientation seen in experiments, indicating that the elastic stresses arising from coherent γ/γ′ interfaces are important for morphological evolution during creep. The results also indicate that the narrow γ channels between γ′ precipitates are energetically favored, and provide an explanation for the experimentally observed directional coarsening that occurs without any applied stress.Download high-res image (189KB)Download full-size image

We extend the existing framework [1–4] of two-point spatial correlations to allow for the quantification, analyses, interpretation and visualization of microstructure coarsening measured by time-resolved X-ray computed tomography. Specifically, extensions were made to facilitate (i) the incorporation of nonconventional local attributes such as solid-liquid interface, interface curvature, and interface velocity in the description of the local state, and (ii) the efficient computation of bulk spatial correlations when the local attributes are sparsely defined only at special locations in the three-dimensional volume (e.g., solid-liquid interfaces). We have explored multiple variants of spatial correlations, including Pearson correlation coefficients and two-point joint probabilities, and examined their relative merits in providing useful new insights into the coarsening process. Algorithmic enhancements needed to carry out these computations on the large datasets produced in the experiments are also described. The results demonstrate the remarkable ability of these new protocols in automated (unbiased) capture of the four-fold symmetry of the dendritic microstructure, and in providing quantitative and reliable estimates of the characteristic lengths associated with the dendritic microstructure (including the secondary and tertiary dendrite arm spacings, secondary dendrite arm diameter, and the solute diffusion length). These estimated quantities agree well with the direct measurements from the microstructure. The results also indicate that interfaces with high negative and near-zero mean curvature (H) have long range spatial auto-correlations, whereas all values of the interfacial normal velocity (V) are only auto-correlated in the short range in space. For mid-range (positive) values of H and non-extreme values of V, the spatial distributions are essentially random.Download high-res image (443KB)Download full-size image

A twin-plane based nanowire growth mechanism is established using Au catalyzed Ge nanowire growth as a model system. Video-rate lattice-resolved environmental transmission electron microscopy shows a convex, V-shaped liquid catalyst-nanowire growth interface for a ⟨112⟩ growth direction that is composed of two Ge {111} planes that meet at a twin boundary. Unlike bulk crystals, the nanowire geometry allows steady-state growth with a single twin boundary at the nanowire center. We suggest that the nucleation barrier at the twin-plane re-entrant groove is effectively reduced by the line energy, and hence the twin acts as a preferential nucleation site that dictates the lateral step flow cycle which constitutes nanowire growth.

We report thermodynamically controlled multi-segmented nanowires for phonon-glass, electron-crystal materials prepared by on-film formation of nanowires (OFF–ON), which is capable of growing high-quality single-crystalline Bi nanowires by the subsequent sputtering of Te onto the Bi nanowires and by post-annealing. The Bi/Bi14Te6 multi-segmented nanowires exhibit lower thermal conductivities than BixTey compounds, while the electrical conductivities of Bi nanowires and multi-segmented nanowires are similar.

The microstructural evolution of equiaxed dendritic Al–Cu microstructures of two different solid volume fractions (46% and 72%) during isothermal coarsening was studied. We found that the microstructures evolved self-similarly, both morphologically and topologically, while the inverse specific surface area increased as the cube root of time. The differences in scaled morphologies and topologies could be attributed to the difference in solid volume fraction, with the higher volume fraction sample exhibiting a more compact interface shape distribution and a smaller scaled genus.

A general two-dimensional faceted model that accounts for capillarity and deposition of an AxB 1−x alloy is developed for the growth of the shell on a hexagonal core. With this model, the surface alloy segregation and morphological evolution in the processes of the faceted core-shell nanowire growth are studied both analytically and numerically. Mechanisms of formation of Al-rich stripes along {112} facets and Al-poor quantum dots/wires at the apices of {112} facets are identified. More specifically, it is found that diffusion tends to move the atoms from {112} facets to {110} facets. The formation of Al-rich stripes along the {112} facets is due to the large ratios of mobilities of Al atoms and Ga atoms on {112} facets, even though Al atoms diffuse slower than Ga on the {110} facets. In addition, the difference of interaction parameters in the enthalpy on different facets can also lead to lines of enhanced concentration of Al behind {112} facets. If the attachment rates of Al on the {112} facets are smaller than that on {110} facets, Al-poor dots will grow at the end of the Al-rich stripes because the growth process switches from diffusion dominant to deposition dominant when the size of the nanowire gets large. Moreover, influences of different parameters on the distribution of concentrations of the atoms in the shell are investigated in details.

Core-shell nanowires with radial heterostructures hold great promise in photonic and electronic applications and controlling the formation of these heterostructures in the core-shell configuration remains a challenge. Recently, GaAs nanowires have been used as substrates to create AlGaAs shells. The deposition of the AlGaAs layer leads to the spontaneous formation of Al-rich stripes along certain crystallographic directions and quantum dots/wires near the apexes of the shell. A general two-dimensional model has been developed for the motion of the faceted solid-vapor interfaces for pure materials that accounts for capillarity and deposition. With this model, the growth processes and morphological evolution of shells of nanowires around hexagonal cores (six small facets {112} in the corners of six equivalent facets {110}) are investigated in detail both analytically and numerically. It is found that deposition can yield facets that are not present on the Wulff shape. These small facets can have slowly time-varying sizes that can lead to stripe structures and quantum dots/wires depending on the balances between diffusion and deposition. The effects of deposition rates and polarity (or asymmetry) on planes {112} on the development of the configurations of nanowires are discussed. The numerical results are compared with experimental results giving almost quantitative agreement, despite the fact that only pure materials are treated herein whereas the experiments deal with alloys.

We report thermodynamically controlled multi-segmented nanowires for phonon-glass, electron-crystal materials prepared by on-film formation of nanowires (OFF–ON), which is capable of growing high-quality single-crystalline Bi nanowires by the subsequent sputtering of Te onto the Bi nanowires and by post-annealing. The Bi/Bi14Te6 multi-segmented nanowires exhibit lower thermal conductivities than BixTey compounds, while the electrical conductivities of Bi nanowires and multi-segmented nanowires are similar.